As a lighting device adapted for lighting a light source composed of LED elements (hereinafter referred to as “LED lighting device”), there has been proposed such a lighting device that includes a chopper circuit so as to adjust (dim) the luminance of the light source. JP2002-231471A discloses a lighting device that can adjust the electric current flowing through an LED light source (hereinafter referred to as “LED current”) so as to dim the LED light source by means of PWM control method. In the PWM control method, the duty ratio of a switching element included in the chopper circuit is variably controlled to adjust the LED current, so that the LED light source is lit in a desired luminance. JP2009-301876A discloses a lighting device that utilizes a first dimming signal and a second dimming signal. The first dimming signal is used for determining a dimming level, and the second dimming signal is used for determining a dimming curve. This lighting device is configured to select, based on the second dimming signal, a desired dimming curve from among a plurality of dimming curves stored in a circuit.
JP2010-40400A discloses a lighting device including a chopper circuit and a power factor corrector connected at an input side of the chopper circuit. This lighting device is configured to terminate the operation of the power factor corrector when light output of an LED element becomes lower than a predetermined level. This lighting device thereby can reduce the flicker of the LED element.
In a lighting device including a chopper circuit, energy is charged in an inductor during an ON period of the switching element, and the energy is discharged to flow an electric current during an OFF period of the switching element. The electric current varies in inverse proportion to the inductance of the inductor. Therefore, if the switching frequency is set at a low level (e.g., less than 40 [kHz]) in the lighting device which is configured to adjust the LED current based on the PWM control method, the lighting device has been required to employ a large-sized inductor with large inductance in order to reduce such a time period in which the electric current does not flow through the inductor in the OFF period.
Furthermore, when the switching frequency is set around 30 [kHz] to 40 [kHz], ripple components emerge on a waveform of the LED current to cause a flickering of light emitted by the LED element. This is likely to interfere with infrared signals emitted by a remote controller provided in another equipment. Therefore, to reduce the ripple component, a smoothing capacitor, which is to be connected in parallel with the LED element, has been required to have a large capacity.
For downsizing the inductor and/or the smoothing capacitor, there has been proposed such an LED lighting device that controls the switching element at a high-frequency. However, if the switching frequency of the switching element is set high, the ON period of the switching element may significantly be shortened (e.g., substantially 0) when the dimming level is decreased and the LED current is reduced. As a result, in the LED lighting device with a high switching frequency, when the dimming level is set low, it may be difficult to control the switching operation stably due to a delay time occurred in a control circuit for controlling the switching operation, a performance limit of the lighting device for driving the switching element, a delay time occurred in a gate-driver, or the like. That is, the conventional LED lighting device may be hard to perform stable dimming control when the dimming level is comparatively low.